Before you start replacing tubes lets determine how much of a problem there is. For both tubes to suddenly start leaking is very unusual. All bike tires lose air and it is not uncommon to lose 10psi is a few days. How much are the tubes deflating and over what time does it occur.

Another consideration is that if you haven't successfully replaced tubes before then please look at one of the Youtube videos and read up on how to do it. Also you will need a set of tire levers...not screwdrivers.